sampler.py

"""
At most cases, we have non-stream dataset, which means we can random access it with __getitem__, and we can get the length of the dataset with __len__.

This suffices for a sampler. We implemente sampler as iterable of valid indices. By valid, we mean 0 <= index < N, where N is the length of the dataset. We then collect several indices within a batch and use it to collect examples from the dataset with __getitem__. Then collate this examples to form a batch.

So the sampler is only responsible for generating valid indices.
"""


import numpy as np
import random

class Sampler(object):
    def __init__(self, data_source):
        pass

    def __iter__(self):
        # return a iterator of indices
        # or a iterator of list[int], for BatchSampler
        raise NotImplementedError


class SequentialSampler(Sampler):
    def __init__(self, data_source):
        self.data_source = data_source
    
    def __iter__(self):
        return iter(range(len(self.data_source)))

    def __len__(self):
        return len(self.data_source)


class RandomSampler(Sampler):
    def __init__(self, data_source, replacement=False, num_samples=None):
        self.data_source = data_source
        self.replacement = replacement
        self._num_samples = num_samples

        if not isinstance(self.replacement, bool):
            raise ValueError("replacement should be a boolean value, but got "
                             "replacement={}".format(self.replacement))

        if self._num_samples is not None and not replacement:
            raise ValueError("With replacement=False, num_samples should not be specified, "
                             "since a random permutation will be performed.")

        if not isinstance(self.num_samples, int) or self.num_samples <= 0:
            raise ValueError("num_samples should be a positive integer "
                             "value, but got num_samples={}".format(self.num_samples))

    @property
    def num_samples(self):
        # dataset size might change at runtime
        if self._num_samples is None:
            return len(self.data_source)
        return self._num_samples

    def __iter__(self):
        n = len(self.data_source)
        if self.replacement:
            return iter(np.random.randint(0, n, size=(self.num_samples,), dtype=np.int64).tolist())
        return iter(np.random.permutation(n).tolist())

    def __len__(self):
        return len(self.data_source)


class SubsetRandomSampler(Sampler):
    r"""Samples elements randomly from a given list of indices, without replacement.
    Arguments:
        indices (sequence): a sequence of indices
    """

    def __init__(self, indices):
        self.indices = indices

    def __iter__(self):
        return (self.indices[i] for i in np.random.permutation(len(self.indices)))

    def __len__(self):
        return len(self.indices)


class PartialyRandomizedSimilarTimeLengthSampler(Sampler):
    """Partially randmoized sampler, implemented as a example sampler
    1. Sort by lengths
    2. Pick a small patch and randomize it
    3. Permutate mini-batchs
    """

    def __init__(self, lengths, batch_size=4, batch_group_size=None,
                 permutate=True):
        _lengths = np.array(lengths, dtype=np.int64) # maybe better implement length as a sort key
        self.lengths = np.sort(_lengths)
        self.sorted_indices = np.argsort(_lengths)

        self.batch_size = batch_size
        if batch_group_size is None:
            batch_group_size = min(batch_size * 32, len(self.lengths))
            if batch_group_size % batch_size != 0:
                batch_group_size -= batch_group_size % batch_size

        self.batch_group_size = batch_group_size
        assert batch_group_size % batch_size == 0
        self.permutate = permutate

    def __iter__(self):
        indices = np.copy(self.sorted_indices)
        batch_group_size = self.batch_group_size
        s, e = 0, 0
        for i in range(len(indices) // batch_group_size):
            s = i * batch_group_size
            e = s + batch_group_size
            random.shuffle(indices[s: e]) # inplace

        # Permutate batches
        if self.permutate:
            perm = np.arange(len(indices[:e]) // self.batch_size)
            random.shuffle(perm)
            indices[:e] = indices[:e].reshape(-1, self.batch_size)[perm, :].reshape(-1)

        # Handle last elements
        s += batch_group_size
        #print(indices)
        if s < len(indices):
            random.shuffle(indices[s:])
        
        return iter(indices)

    def __len__(self):
        return len(self.sorted_indices)


class WeightedRandomSampler(Sampler):
    r"""Samples elements from ``[0,..,len(weights)-1]`` with given probabilities (weights).
    Args:
        weights (sequence)   : a sequence of weights, not necessary summing up to one
        num_samples (int): number of samples to draw
        replacement (bool): if ``True``, samples are drawn with replacement.
            If not, they are drawn without replacement, which means that when a
            sample index is drawn for a row, it cannot be drawn again for that row.
    Example:
        >>> list(WeightedRandomSampler([0.1, 0.9, 0.4, 0.7, 3.0, 0.6], 5, replacement=True))
        [0, 0, 0, 1, 0]
        >>> list(WeightedRandomSampler([0.9, 0.4, 0.05, 0.2, 0.3, 0.1], 5, replacement=False))
        [0, 1, 4, 3, 2]
    """

    def __init__(self, weights, num_samples, replacement):
        if not isinstance(num_samples, int) or num_samples <= 0:
            raise ValueError("num_samples should be a positive integer "
                             "value, but got num_samples={}".format(num_samples))
        self.weights = np.array(weights, dtype=np.float64)
        self.num_samples = num_samples
        self.replacement = replacement

    def __iter__(self):
        return iter(np.random.choice(len(self.weights), size=(self.num_samples, ),  
                                     replace=self.replacement, p=self.weights).tolist())

    def __len__(self):
        return self.num_samples


class BatchSampler(Sampler):
    r"""Wraps another sampler to yield a mini-batch of indices.
    Args:
        sampler (Sampler): Base sampler.
        batch_size (int): Size of mini-batch.
        drop_last (bool): If ``True``, the sampler will drop the last batch if
            its size would be less than ``batch_size``
    Example:
        >>> list(BatchSampler(SequentialSampler(range(10)), batch_size=3, drop_last=False))
        [[0, 1, 2], [3, 4, 5], [6, 7, 8], [9]]
        >>> list(BatchSampler(SequentialSampler(range(10)), batch_size=3, drop_last=True))
        [[0, 1, 2], [3, 4, 5], [6, 7, 8]]
    """

    def __init__(self, sampler, batch_size, drop_last):
        if not isinstance(sampler, Sampler):
            raise ValueError("sampler should be an instance of "
                             "Sampler, but got sampler={}"
                             .format(sampler))
        if not isinstance(batch_size, int) or batch_size <= 0:
            raise ValueError("batch_size should be a positive integer value, "
                             "but got batch_size={}".format(batch_size))
        if not isinstance(drop_last, bool):
            raise ValueError("drop_last should be a boolean value, but got "
                             "drop_last={}".format(drop_last))
        self.sampler = sampler
        self.batch_size = batch_size
        self.drop_last = drop_last

    def __iter__(self):
        batch = []
        for idx in self.sampler:
            batch.append(idx)
            if len(batch) == self.batch_size:
                yield batch
                batch = []
        if len(batch) > 0 and not self.drop_last:
            yield batch

    def __len__(self):
        if self.drop_last:
            return len(self.sampler) // self.batch_size
        else:
            return (len(self.sampler) + self.batch_size - 1) // self.batch_size